The eyes are the window into the soul - or at least the mind, according to a new paper published in Perspectives on Psychological Science, a journal of the Association for Psychological Science. Measuring the diameter of the pupil, the part of the eye that changes size to let in more light, can show what a person is paying attention to. Pupillometry, as it's called, has been used in social psychology, clinical psychology, humans, animals, children, infants - and it should be used even more, the authors say.